The invention concerns an open-loop and closed-loop control method for an internal combustion engine with a common rail injection system, in which the rail pressure is subject to closed-loop control during normal operation.
In a common rail system, a high-pressure pump pumps the fuel from a fuel tank into a rail. The admission cross section to the high-pressure pump is determined by a variable suction throttle. Injectors are connected to the rail. They inject the fuel into the combustion chambers of the internal combustion engine. Since the quality of the combustion is decisively determined by the pressure level in the rail, this pressure is automatically controlled. The closed-loop high pressure control system comprises a pressure controller, the suction throttle with the high-pressure pump, and the rail as the controlled system. Typically, the pressure controller is realized as a PID controller or a PIDT1 controller, that is, it comprises at least a proportional component (P component), an integral component (I component), and a differential component (D component). In this closed-loop high pressure control system, the controlled variable is the pressure level in the rail. The measured pressure values in the rail are converted by a filter to an actual rail pressure and compared with a set rail pressure. The control deviation obtained by this comparison is converted to a control signal for the suction throttle by the pressure controller. The control signal corresponds, e.g., to a volume flow in liters/minute units. The control signal is typically electrically generated as a PWM signal (pulse-width-modulated signal). The closed-loop high pressure control system described above is disclosed by DE 103 30 466 B3.
To protect against an excessively high pressure level, a passive pressure control valve is installed in the rail. If the pressure level is too high, the pressure control valve opens to conduct fuel from the rail back into the fuel tank.
The following problem can arise under practical conditions: a load reduction is immediately followed by an increase in engine speed. At a constant set speed, an increasing engine speed causes an increase in the magnitude of the speed control deviation. A speed controller responds to this by reducing the injection quantity as a correcting variable. A smaller injection quantity in turn causes less fuel to be taken from the rail, so that there is a rapid increase in the pressure level in the rail. The situation is further complicated by the fact that the output of the high-pressure pump depends on the engine speed. An increasing engine speed means a higher pump output, and this produces a further increase in pressure in the rail. Since the high pressure control system has a relatively long response time, the rail pressure can continue to rise until the pressure control valve opens, e.g., at 1,950 bars. This causes the rail pressure to drop, e.g., to a value of 800 bars. At this pressure level, an equilibrium state develops between fuel pumped in and fuel removed. This means that, despite the opened pressure control valve, the rail pressure does not drop further. The pressure control valve does not close again until the speed of the internal combustion engine is reduced. Therefore, the unexpected opening of the pressure control valve after a load reduction is a problem.
The German Patent Application with the official file number DE 10 2004 023 365.9, for which a prior printed publication has not yet appeared, also describes a closed-loop pressure control system for a common rail system. In this closed-loop pressure control system, in addition to the first filter, a second filter is located in the feedback path. The second filter has a smaller time constant and a smaller phase delay than the first filter. The actual rail pressure determined by the second filter is used for the calculation of the controller components. This results in an improved dynamic response of the closed-loop high pressure control system in the event of a load reduction.
It remains critical, however, that the control signal or the PWM signal is strongly limited by the electrical characteristics of the electronic control unit, e.g., maximum continuous current and dissipation of the output transistor. This means that, at a large control deviation, although the pressure controller computes a maximum correcting variable, this variable ultimately can be converted to a PWM signal with only, e.g., 22% pulse to no-current ratio. A permanently applied higher PWM value would cause deactivation of the output stage of the electronic control unit.